Interline transfer architecture for solid-state image sensors is well known in the prior art. Examples of such prior art image sensors include U.S. Pat. No. 4,521,797 by Oda, et al., U.S. Pat. No. 5,084,749 by Losee et al, and U.S. Pat. No. 5,235,198 by Stevens, et al., and U.S. Pat. No. 4,527,182 by Ishihara, et al. In interline transfer devices, an array of photodetectors, such as photodiodes or photocapacitors collect and store photo-generated charge. A transfer gate associated with each photodiode enables the photo-generated charge to be transferred to a vertical shift register. This transfer typically occurs at the end of the image capture period. The vertical shift registers then transfer the charge to a horizontal shift register, which then transfers the photo-generated charge, pixel-by-pixel to an output structure. Referring to FIG. 1, a top plan view of a portion of an interline transfer image sensor 5 shows four pixels 10 each with a photo-detector 20, transfer gate 30, and a portion of the vertical shift register 40.
Interline transfer image sensors have the ability to capture a new image while at the same time transferring the charge associated with the previous image capture to the output amplifier. This can be accomplished because the photo-detector is separate from the charge transfer areas. During the time required for the read-out operation, light is still incident on the photodiode or photo-capacitor, and photo-generated charge may be collected and stored for the following frame. In addition, the vertical shift registers 40, or vertical charge coupled device (VCCD), used for charge transfer, are covered with a lightshield 50 to prevent light from entering the silicon portion of the VCCD 40 and generating charge. The lightshield 50 is typically made from a metal layer such as aluminum, tungsten, or tungsten silicide.
While the lightshield 50 prevents most light from entering the silicon portion of the VCCD 40, it is not perfect, and several improvements have been disclosed over the years to reduce the exposure of the silicon in the VCCD 40 to light. Such exposure will produce spurious signals in the detected image, thus degrading the performance of the device. This spurious signal has been called image smear in the prior art literature. N. Teranishi and Y. Ishihara in IEEE Transactions on Electron Devices, ED-34, 1052, (1987) describe sources of smear and some approaches to reduce smear where aluminum is used as the lightshield material. Smear can be reduced by decreasing the insulator thickness between the lightshield material and the polysilicon gate electrode. To reduce that thickness, D. Losee and M. Mehra in U.S. Pat. No. 5,084,749 introduce the use of WSix as lightshield material. In addition, this patent describes the use of a silicon oxide film doped with boron and/or phosphorus that is annealed and flowed on top of the lightshield 50 which results in improved topography for color filter array application. In addition, Losee et al. describe the shortcoming of WSix where x=2 or x>2 because the transmission of such films allows too much light into the shift register, and also the advantages of WSix where x<2 for improved opacity.
In large-area interline CCD devices, the high resistance of doped polysilicon electrodes combined with the large capacitance of the shift register limits the frame rate for these sensors. Because both WSix and aluminum have lower resistivity than polysilicon, several schemes have been disclosed to shunt or strap the polysilicon electrodes using the lightshield layer. Referring to FIG. 2, Nichols et al. in “Single Chip Color HDTV Image Sensor with Two Polysilicon Levels and with WSix Lightshield Used for Strapping Vertical Gates”, 1992 International Electron Devices Meeting Technical Digest, 101, (1992) describe the incorporation of contact holes 60 cut into the dielectric 70 separating the lightshield and the polysilicon electrodes of the shift register to provide a lower resistance path for current to drive the shift register electrodes. A similar approach, but with a tungsten lightshield is described by K. Orihara et al. in “New Shunt Wiring Technologies for High Performance HDTV CCD Image Sensors”, 1992 International Electron Devices Meeting Technical Digest, 105, (1992). K. Orihara prefers to use tungsten instead of aluminum because the aluminum shunt wiring requires a buffer layer of polysilicon between the aluminum and polysilicon electrodes to avoid formation of potential shifts which degrade charge transfer efficiency. Kamisaka in U.S. Pat. No. 5,432,363 also reports this degradation when aluminum shunt wiring is connected directly to the polysilicon electrode.
It is advantageous to provide a smooth surface upon which the aluminum-wiring layer is deposited and the subsequent color filter array is fabricated. As was described earlier, this is often accomplished through deposition of a boron and/or phosphorus containing silicon oxide film on top of the lightshield and photoactive regions, then annealing the structure at 800–950 C. which causes the film to reflow and provide a smoother top surface. This reflowed film can also be used to help focus light into the photoactive region. Aluminum as a lightshield material cannot be used in this approach because aluminum melts at temperatures below the reflow annealing temperature.
Referring to FIG. 3, Kamisaka discloses a lightshield 80 consisting of two layers, a polysilicon layer 90 below a refractory metal or refractory metal silicide 95. Because the high-temperature anneal is desired for reflow of a doped oxide deposited over the lightshield, the bottom layer of polysilicon is used to prevent a degradation of charge transfer efficiency due to interaction of the refractory metal with the polysilicon gate electrode at reflow anneal temperatures. Y. Maruyama and D. Sugimoto in U.S. Pat. No. 6,504,188 state that the contact resistance between the refractory metal lightshield film and the polysilicon electrode increases due to heat treatment of the dielectric film covering the lightshield which is in direct contact with the polysilicon gate electrode. Therefore they describe the use of a lightshield consisting of polysilicon layer below a refractory metal nitride or oxide, as well as a refractory metal layer.
The addition of a polysilicon layer below the lightshield adds additional processing steps such as the deposition of polysilicon, heavy phosphorus doping of the polysilicon layer using POCl3 or high-dose implantation. In some cases, separate photolithography and etching steps separate from the refractory lightshield patterning steps are necessary. This polysilicon layer also increases the topography of the device which may cause difficulties in depositing and patterning of later layers.